Automobile energy consumption is meaningfully expressed as
energy/distance. In SI units, that would be J/m, which simplifies to
newtons. While probably not of great practical value, it is the SI
unit, and it is intuitive. It is equivalent to the force that would be
pushing your car if the fuel your car used had been converted perfectly
into mechanical energy.
For example, on my last tank I managed to coax 40.2 mpg out of my
Corolla. That works out to:
40.2 miles = 64 700 m
1 gallon of unleaded gasoline = 131 MJ
Thus, I averaged 131 000 000 / 64 700 = 2 025 N.
Since most people don't have a good feel for Newtons, on earth that's
the weight of 206 kg, which seems rather hefty.
To keep it simple, the conversion factor would be:
Energy consumption (in Newtons) = 81 300 / fuel economy in MPG
My wife's car averages about 3,700 N, which is the weight of 380 kg!
Those who prefer distance/energy would use inverse newtons instead of
newtons. Not as intuitive.
If, on the other hand, we care more about the volume of fuel used rather
than energy, then:
1 gallon of gasoline = 0.003785411784 meters cubed
40.2 miles = 64 700 m
Thus, my fuel consumption was 0.003785411784 / 64 700 = 0.0586 mm
squared, which seems like an amazingly thin thread of gasoline.
Would someone check over my reckoning to make sure I haven't made a mistake?
J.
